System for low-frequency transmission of radiant energy

ABSTRACT

A communication system utilizing a highly directive lowfrequency transmission of radiant energy which is generated by directing a beam of radiant energy at a first high frequency and at a second high frequency into a nonlinear transmission medium. By virtue of an interaction in the nonlinear medium between the energies at the first and the second high frequencies, a low frequency beam of radiation is produced having a directivity pattern comparable to that of the energy radiated at the high frequencies. The system is particularly useful for sonic examination of the seabed for sub-oceanic strata and buried objects such as pipes, in which case a receiving hydrophone and display are utilized to plot a graphical display of echoes reflected from the strata.

United States Patent [1 1 Chramiec et a].

[ Jan. 15, 1974 SYSTEM FOR LOW-FREQUENCY [541 3,613,069 l/l97l Cary, Jr.340/3 R TRANSMISSION OF RADIANT ENERGY [75] Inventors: Mark A. Chramiec,Newport, R.l.; Primary Farley William L Konrad Niantic ConnAtt0rneyMllton D. Bartlett Ct al. v

[73] Assignee: Raytheon Company Lexington,

Mass. [57] ABSTRACT 22 Filed; No 3 1 7 A communication system utilizinga highly directive low-frequency transmission of radiant energy which is[21] APPL 304,884 generated by directing a beam of radiant energy at a lRelated s Application Data first high frequency and at a second highfrequency [62] Division of Ser No In 218 Feb 1 971 into a nonlineartransmission medium. By virtue of an abandoned interaction in thenonlinear medium between the energies at the first and the second highfrequencies, a low 52 U S Cl N 340/3 R 340 3 FM 343/100 CL frequencybeam Of radiation iS produced having a di- 51 Int. Cl. G015 9/68rectivity Patter" comparable to of the energy [581 Field of Search. I340/1 R 3 FM, 3 R, diated at the high frequencies. The system isparticu- 340/ 343/100 181/05 R, 05 J 0.5 larly useful for sonicexamination of the seabed for AG, 05 A sub-oceanic strata and buriedobjects such as pipes, in which case a receiving hydrophone and displayare [56] References Cited utilized to plot a graphical display of echoesreflected UNITED STATES PATENTS fmm 3,510,833 5/l970 Turner 340/5 R X 26Claims, 5 Drawing Figures '1 SOURCE I 52 l I94 KHz l 56 I l 54 l rmuucGATE 5a 62 44 48 50 22 SOURCE FILTER 72 z AMPL TRANSMIT 467 KHZ' X 66!KHz 3 l TRANSDUCER t X FILTER I ,2 l 76v SOURCE X FILTER 64 206ml 46 Il2 KHZ 455m: I

68 7 1/21 I I 66 L70 I REF 0 Q26 MODULATOR (m CHIRP) :IZKHZ4 CORRELATORRECEIVER RECE'V'NG I -73 TRANSDtfER 24 IL I44 SWITCH 74 [46 Il 30 BEPLAY re SHIPS LOC TlON DATA PATENTEU JAN 15 I974 sumlum Pmmmm 1 5 m43786.405 SHEET 2 BF 4 SOURCE I94 KHZ 44 a y 48 50 I 22 467 KHz 66l KHz lX 4F|LTER l 206KHz 46 I /2 SOURCE X FILTER IZKHZ 68 45KH2 64 N21 I 66 IL70 REF //0 6 26 MODULATOR H ISSKHZ i (FM CHIRP) m IIZKHz AE RECEIVINGTRANSDUCER 73 j I 24 74 I46 l DISPLAY 7a 1 SHIP-s 2 LOCATION DATA SYSTEMFOR LOW-FREQUENCY TRANSMISSION OF RADIANT ENERGY RELATED APPLICATIONBACKGROUND OF THE INVENTION In the past, examination of the seabed forsuboceanic strata and for objects buried in the sea floor has beenparticularly difficult due to the fact that high energy sonic radiationsare required to provide high resolution narrow beam patterns. However,these high frequencies either tend to reflect off the first interfacebetween the ocean waters and sub-oceanic strata such as sand and gravelat the ocean bottom, or are rapidly attenuated as they propagate throughsuch strata. Consequently, little if any high resolution data canthereby be provided relative to sub-bottom strata or buried objects. Byway of contrast, low frequency sonic radiations readily penetratesub-bottom strata with sufficient remaining energy to reflect offsubmerged objects such as buried pipes. However, a problem arises in theuse of such low frequency sonic radiations in that generally it isimpractical to provide a transducer of sufficient physical size togenerate a beam of such radiation having a sufficiently narrow beamwidth to permit high resolution examination of the ocean bottom.

SUMMARY OF THE INVENTION In accordance with the inventionthere isprovided a means for generating a narrow beam of low frequency radiationby utilization of a relatively small sized high frequency transducer, orradiating antenna. The transducer is excited with energy ata first andat a second 'high frequency. The transducer being linear transmits twonearly identically dimensioned beams of high frequency energy, one ofthese beams having a directivity pattern associated with the wavelengthof energy at the first high frequency and the second beam having a directivity pattern associated with the wavelength of radiation at thesecond high frequency. The two beams propagate through a nonlinearmedia, in this case two beams of sonic energy traversing a region of theocean waters, and interact to generate energy which radiates outwardlyfrom the interaction region at a frequency equal to the difference ofthe first and the second high frequencies. It is believed that theinteraction region, being many times larger than'the transducer, is ableto form a narrow beam of radiation due to its relatively large sizecompared to a wavelength of the low frequency radiation. Thus, there isproduced a low frequency narrow beam of sonic radiation which penetratesthe ocean bottom and is thus able to extract high resolution data ofsub-bottom layers and buried objects against which the sound impinges.Reflections from these objects are received by a suitable means such asa hydrophone and are presented on a display similar to that utilized insonar'systems-for a graphical portrayal of the ocean bottom and objectssubmerged therein.

BRIEF DESCRIPTION OF THE DRAWINGS The aforementioned aspects and otherfeatures of the invention are explained in the following descriptiontaken in connection with the accompanying drawings wherein:

FIG. 1 is a stylized pictorial representation of a ship carrying theinvention for gathering data of the ocean bottom;

FIG. 2 is a block diagram of the invention;

FIG. 3 is a typical display of an ocean bottom profile taken with asonar system embodying the invention;

FIG. 4 is a detailed block diagram of a receiver and correlator utilizedin the invention; and

FIG. 5 is a block diagram of an alternative embodiment of the invention.I

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1 there isshown a stylized pictorial representation of the communication system 10utilizing a nonlinear medium 12, herein the ocean waters, for convertingradiant energy at relatively high frequencies indicated by arrows 14 toradiant energy at a ,low frequency indicated by arrows 16. A ship 18carries 32A and 32B of the ocean 32, the interface 34 between the ocean32 and the ocean bottom 28, marinelife 36 and, more particularly, thepipe 26. The display can also show sub-bottom strata useful for locatinggeological surveying. The location of the ship 18 may be determined bywell-known means such as an inertial navigator or, as seen in theFigure, by a plurality of stations 38 on the shore 40 which transmitsand receives signals from the ship's antenna 42 to effect atriangulation procedure which continuously monitors the location of theship 18. This location information is utilized in a wellknown manner bythe display 30 to provide a map of the ocean bottom 28 and to form apictorial representation and the location of the pipe 26.

Referring now to FIG. 2 there isshown a block diagram of the signalgenerator 20 and its interconnections with the transmitting transducer22, the receiving transducer 24 and the display 30. The signal generator20 provides electrical energy at two frequencies, shown by way ofexample as 194 KHz on line 44 and 206 KHz on line 46, which are thensummed together by a summing circuit 48 and amplified by amplifier 50 toa power level suitable for conversion into sonic energies at these twofrequencies by means of the transmitting transducer 22. The electricalenergy on line 44 is provided by a signal'source 52, a continuous wave(cw) sinusoidal generator at 194 KHZ, via a gate 54 which is operated bya timing unit 56. The timing unit 56 periodically opens and closes thegate 54 to provide a pulsed cw signal on the line 44. The pulsed signalfrom the gate 54 and a second cw signal at 467 KHz from signal source 58are combined in a multiplier 60, which may be a well-known bridgemodulating circuit, to provide electrical energy at a plurality offrequencies, one of which, 661 KHz, is passed by a filter 62 to asimilar multiplier 64. A third source, signal source 66, provides a cwsignal at 12 KHz which is applied to a multiplier 68 similar tomultiplier 60. Recalling that the transmitting transducer 22 transmitssonic energy at frequencies of 194 KHZ and 206 KHz into the nonlinearmedium 12, the interaction of these high frequency sonic' signals in thenonlinear medium 12 produces a low frequency sonic signal of value 12KHz which reflects off the pipe 26 to be received by the receivingtransducer 24. Thus, it is seen that the frequency, 12 KHz, of thesignal at the receiving transducer 24 is equal to the frequency of thesignal source 66.

The signals of sources 58 and 66 are applied to the multiplier 68 whichprovides electrical energy at a plurality of frequencies one of which,455 KHZ, is passed by a filter 70 to a multiplier 64. The signals passedby filters 62 and 70 are similarly processed by the multiplier 64 and afilter 72 to provide the signal at 206 KHz on line 46. It is noticedthat the signal on line 46 is continuously applied by the summingcircuit 48and the amplifier 50 to the transmitting transducer 22 whilethe signal on line 44 is pulsed. Accordingly, the low frequency signalsat 12 KHz directed to the pipe 26 is a pulsed signal having the samepulse width as the signal on line 44.

The signalsreceived from the receiving transducer 24 are applied by areceiver 73 (to be described hereinafter) and a switch 74 to the display30. The display 30 is triggered in a well-known manner by signals online 76 from the timing unit 56 so that the time delays experienced bysignals propagating from the transmitting transducer 22 through theocean 32 of FIG. 1- appear on the display 30 as the distances from theship 18 to the interface 34 and the pipe 26. Successive passes of theship 18 across the pipe 26 provide the aforementioned mapping of theocean bottom by means of the ship's location data 78 (obtained by thetriangulation or from an inertial navigator) and by the interconnectionof the ships location data 78 with the display 30.

It may also be desirable to modulate the low frequency sonic signaldirected at the pipe 26. This is effected by modulator 80 which appliesa voltage signal having a predetermined waveform to the signal source66. Thus, for example, if the signal source 66 is a voltcomprises asummer 100, an amplifier 102, a multiplier 104, a filter 106 and alimiter 108 A signal from the reage tunable oscillator, the phase orfrequency of the signal of source 66 may be phase or frequency modulatedto provide, for example, an FM chirp signal which results in a similarFM chirp modulation being directed into the pipe 26. The filter 70 and72 are provided in a well-known manner with sufficient bandwidth to passthe signal modulation. In this case, range data representing thedistanceof the pipe 26 from the ship 18 is obtained by correlating the signal atthe receiving transducer 24 with a stored replica 84 of the modulatedsignal at correlator 82 as will be described hereinafter with reference.to FIG. 3. The data is displayed by switching the switch I74 to passdata from the correlator 82 to the display 30.

FIG. 3 shows a typical mapping of the ocean bottom showing an objectsubmerged in the ocean bottom. In-, terfaces between the waters of theocean, the first bottom and subbottom strata are indicated by numeral86, and a submerged object is indicated by numeral 88. The ocean depthor distance from the ship 18 (of FIG. I) to the submerged object 88 isindicated by the vertical axis 90, and the distance along the oceanbottom is indicated by the horizontal axis 92. v

Referring now to FIG. 4 there is shown a detailed block diagram of thereceiver 73 and the correlator 82 of FIG. 2 including theirinterconnections with other components of the system of FIG. 2. Thereceiver 73 the amplifier 102 which amplifies the signal to a suit-'able level for processing by the multiplier 104. The multiplier 104 is,for example, a well-known diodebridge modulating circuit which combinesa reference signal 110 having a frequency of 15.5 KHzwith the 12 KHzsignal provided by the amplifier 102 to produce a signal at 3.5 KI-Iz.The filter 106 has a sufficiently wide pass band centered at 3.5 KB; topass the signal, a pulsed, frequency-modulated sinusoid, to the limiter108 which then provides a symmetrical clipping effect to the signalthereby transforming it into a signal on line 111 having a substantiallytrapazoidal or rectangular waveform.

The amplifier 50 seen in both FIGS. 2 and 4 which provides the highpowered signal to the transmitting transducer 22 also has a secondoutput on line 1,12 through which is coupled a low powered replica ofthe signal applied to the transmitting transducer 22. The signal on line112 may be provided, by way of example, by extracting a portion of theoutput signal of the amplifier 50 through a large attenuator (notshown). The signal on line 112 will be utilized in a manner to bedescribed for inserting a replica of the l2 KHz signal impinging on thepipe 26 of FIG. 2 into the correlator 82. Since the frequency of the lowfrequency signal impinging on the pipe 26 is equal to the difference ofthe two high frequencies of the signals applied to the transmittingtransducer 22, the signals on line 112 are applied to a nonlinearelement 114, such as a diode, which results in a low frequency signal at12 KHz which is passed by filter 116, having a bandwidth at least asgreat as that of filter 106, along line 1 17 to the summer 100. Thus,prior to the reception of the echo from the pipe 26 by the receivingtransducer24, a 12 KHz replica obtained via the nonlinear element 114 isapplied to the amplifier 102 with the result that the receiver 73 nowprovides initially a replica of the low'frequency signal impinging uponthe pipe 26 which is followed subsequently by an echo of the lowfrequency signal from the pipe 26.

Referring again to FIG. 2 the signal source 66 comprises a variablefrequency oscillator centered at 3.5 KHz (notshown in the Figures) and amultiplier and filter similar to the multiplier 104 and filter 106 fortranslating the signal from the frequency of 3.5 KHz to a frequency of12 KHz by mixing the 3.5 KI-Iz signal with the reference signal 110 in,a well-known manner.

. The signal frequency of 3.5 KHz is utilized since this corresponds tothe-system data rate of 3.5 KHz. It is also apparent that, withreference to FIG. 4, the 12 KHz replica provided by the filter 116 maybe provided, alternatively in a more simple manner by connecting theoutput of the signal source 66 of FIG. 2 directly to the summer however,the replica provided by the filter 116 is more advantageous in that itcan be more readily made to closely approximate the low frequency signalactually impinging upon the pipe 26 of FIG. 2.

The correlator 82 may be of a standard form such as that described inU.S. Pat. No. 2,958,039 which issued to V.C. Anderson on Oct. 25, 1960or in U.S. Pat. No. 3,488,635 which issued to T.P. Sifferlin on Jan. 6,1970, or as shown in FIG. 4 comprises a sampler 118 which samples thesignal provided by the limiter 108 at a relatively high sampling ratesuch as 20 [(111, a reference time compressor 120 which is maderesponsive to the replica signal on line 117 by means of a gate 122, asignal time compressor 124 responsive to all signals passed by theamplifier 102, a coincidence detector 126 for indicating thesimultaneous presence of identical samples of compressed signals in thetwo time compressors 120 and 124, and an integrator 128 in the form of alow pass filter suitable for passing a 3.5 KHz signal for intergratingthe output pulses of the coincidence detector 126 to provide anamplitude modulated sinusoidal waveform at 3.5 KHz in which theamplitude represents the degree of correlation between the replica andthe echo signals. The reference time compressor 120 comprises a shiftregister 130 providing serial storage of 1,032 bits and a switch 132 forselectively applying an output signal of the shift register 130 or aninput signal on line 134 provided by the sampler 118. The signal timecompressor 124 similarly comprises a shift register 136 providingserially 1,031 hits of storage and a switch 138 for selectively applyingto the shift register 136 either an output signalof the shift register136 or an input signal on line 134.

Referring now to FIGS. 2 and 4, the pulse width of the low frequencysignal impinging upon the pipe 26 has a value of, for example, 50milliseconds as is provided by the gate 54 in accordance with signalsfrom the timing unit 56. At the same time that the gate 54 is madeconducting, the gate 122 is similarly made conducting by signals alongline 76 from the timing unit 56 so that the replica signal on line 117can pass via sampler 118 into the reference time compressor 120. Thegates 54 and 122 are simultaneously rendered nonconducting by the timingunit 56 so that thereafter signals such as noise or echoes entering thereceiving transducers 24 are excluded by the gate 122 from entering thereference time compressor 120.

The sampling rate, 20 KHz, the sampler 118 is sufficiently high relativeto the pulse center frequency, 3.5 KHz, of the signal on line 1 11 suchthat several samples of the signal are extracted by the sampler 118during a singleperiod of the signal. Each of the samples are in the formof a narrow pulse having a width on the order of nanoseconds.

The two time compressors 120 and 124 operate in essentially-the samemanner so that only the operation of the signal time compressor 124 needbe described. Each of the pulse samples on line 134 pass through switch138 into the shift register 136. A pulse entering the input to the shiftregister 136 is then sequentially shifted along the shift register 136in response to clock pulses on line 76 provided by the timing unit 56.The switch 138 is similarly operated by timing pulses from the timingunit 56. The sampler 118 provides a one bit sample at which a 1represents a positive portion of the waveform on line 111 and arepresents a negative portion of the waveform on line 11 1. Thus thereis provided a sequence of one bit digital numbers emanating from thesampler 118. Each one of these one bit samples is applied to the inputof the shift register 136 and is then shifted down the shift register ata much higher rate than that at which these one bit signals appear online 134. These one bit signals are shifted from stage to stage of theshift register 136 at a clock rate of approximately MHz.

It is readily seen that for a signal on line 111 having a width of 50milliseconds, the sampling rate of 20 KHz provides 1,000 samples of thesignal. The width of the replica signal on line 117 and the samplingrate are adjusted so that there are a total of 1,032 samples obtainedfor-each of the replica signals on line 117. Thus the shift register iscompletely filled with the samples obtained by sampling the replicasignal on line 1 17, while the shift register 136 having one lessstorage stage permits the first sample to overflow, thereby resulting ina precession of the data stored in the shift register 136 relative tothe data stored in the shift register 130. The precession is aided'byswitch 138 which normally conducts samples from the output of the shiftregister 136 to its input, but momentarily switches to admit a sample online 134 and thereby discards from storage a sample admitted 1,031samples earlier. The time required for a sample to circulate onecomplete cycle through the shift register 136 or 130 is smaller than theintersample interval'by an amount of time equal to approximately thewidth of one sample so that a new sample on line 134 can enter into theshift register 136 immediately after the preceding sample hasrecirculated through the switch 138 back into the shift register 136. Atthesampling rate of approximately 20 KHz there is approximately 50microseconds alotted per sample. For 1,032 samples, thesample pulsewidth on line 134 must be less than 50 nanoseconds, for example, in therange of 30 to 40 nanoseconds.

After the shift register 130 has been filled, binary data samples areapplied on line 140 to the coincidence detector 126 at a 20 MHz rate.Similarly, the shift register 136 which has also initially been filledwith the data from the replica signal on line 117 applies binary datasamples on line 142 at a 20 MHz rate to the coincidence detector 126.Since the two shift registers 136 and 130 have been loaded withidentical data at the identical time, the signals on lines 140 and 142are in phase so that the coincidence detector 126 provides output binarysignals at a 20 MHz rate to the integrator 128, each binary signal beinga 1 when there is a coincidence or cophasal relationship between thedata samples on line 140 and .142, and a 0 when the signals on line 140and 142 are out of phase. Accordingly, it is apparent that, initially,as the shift registers 130 and 136 fill up, the coincidence detector 126provides a sequence of ls. Later, after the replica signal on line 117is completed, noise and reverberations from the ocean 32 of FIG. 1appear on the line 111 and are sampled by sampler 118 so that randomlyoccurring samples appear at the input to the signal time compressor 124.Due to the precession of the two time compressors 120 and 124 relativeto each other and also due to the fact that the data stored within thesignal time compressor 124 is slowly being replaced with random samples,coincidences between the signals on lines 140 and 142 occur, in a randomfashion so that the 0's and 1's appearing at the output of thecoincidence detector 126 also begin to occur in a random fashion.

A low amplitude signal results at the output of the integrator 128 inresponse to randomly occurring input pulses to the integrator 128, whilea relatively high amplitude signal results at the output of theintegrator 128 in response to a sequence of 1's emanating from thecoincidence detector 126. it is also apparent that one completeprecession cycle occurs during a time interval equal to the width of areplica signal on line 117. Thus,

' a complete shifting of a received echo relative to the replica storedin the reference time compressor 120 is a signal at the differencefrequency F D the frequency varies from a maximum value to a value ofzero and then returns to its maximum value while the phase of the signalundergoes a 180 phase shift as it passes through the zero frequencypoint. Thus, the bandwidth of the signal at the difference frequency isequal to substantially twice the average or center frequency, the

term "substantially" being used, since at frequencies near zero theconversion efficiency of the water in converting F l and F, into F D isminimal so that in a practical system the energy content of signals at anear zero a submerged object. The signal on line 144 and the signalapplied to the limiter 108 are similar in that each is a sinusoid at afrequency of 3.5KHz with an amplitude related to the strength of an echoobtained from the ocean bottom 28 or the pipe 26 of FIG. 1. These twosignals differ in that the signal on line 144 has a very highsignal-to-noise ratio as compared to the signal on line 146 (from filter106) in those situations where the echo signal strength is weak comparedto the noise environment.

The presence of the sinusoidal signal at a frequency of 3.5 KB: at theoutput of the integrator 128 can be explained as follows. Recalling thata full precession cycle occurs during the duration of a replica signalon line 117, the number of phase reversals between signals on line 140and 142 is equal to the number of cycles of the signal on line 146occurring during the duration of a replica signal on line 117. Thenumber of cycles of phase reversal is equal to the frequency of thesinusoid times the pulse width, and the rate of occurrence of thesephase reversals is accordingly the number of cycles divided by the pulsewidth which is simply the frequency of 3.5 KI-Iz. Thus, even when thereis perfect correlation there is still a periodic occurrence of groups of1's and Os occurring at the output of the coincidence detector 126 whichresults in the sinusoid having the frequency of 3.5 KI-Iz appearing atthe output of the integrator 128. 7

Referring now to FIG. there is shown a block diagram of a signalgenerator 210 which represents an alternative embodiment of the signalgenerator of FIGS. 1 and 2. The signal generator 210 utilizes twosinusoidal waveforms of frequencies F, and F, which are summed togetherby summer 212 and applied via amplifier 114 to the transmittingtransducer 22. The sinusoidal cw signal at the frequency F, is providedby source 216, while the sinusoidal cw signal at the frequency F, isprovided by a source 218 in combination with a well-known frequencymultiplier 220. As shown, by way of example in FIG; 4, a source 218provides-a frequency F,l4 and the frequency multiplier 220 has amultiplication factor of 4 which may be conveniently implemented bymeans of nonlinear diode network in which the fourth harmonic isobtained or, alternatively by means of a pair of serially connectedfrequency doubler circuits. The source 216 is modulated by modula tionsignals provided by modulator 222 to provide, for example, an FM chirpwaveform centered about the frequency F A difference frequency F D F, F,formed by the interactions of the signals at frequencies F 1 and F, inthe water of the ocean 32 reflects off the pipe 26 in the ocean bottom28 and is received by the receiving transducer 24.' Since the frequencymodulation of the signal at F, is centered about the frequency F it isapparent that the difference frequency F momentarily passes throughzero. It is thus seen that in difference frequency is too low to beutilized.

To make maximum use of the'energy content of the signal at thedifference frequency, a correlator 224 correlates the signal received attransducer 24 with a reference or replica applied in the followingmanner. The

signal received at transducer 24 is sampled in a one-bit sampler 226which preserves information relative to the zero crossings of thissignal and stores this data in a shift register 228. In order tofacilitate the sampling at the lower frequencies, the signal from thereceiving transducer 24 is first mixed in mixer 230 with a reference online 232 of frequency value F,/4 so that the signal entering the sampler226 is centered about the frequency of F /4. In a similar way samplingby sampler 234 of a replica signal is provided by means of mixers 236and 238 and a shift register 240. A mixer 236 coincides the signals atthe frequencies F /4 and F, to provide a signal at a freuqency of (%)F,which upon being mixed by mixer 238 with the signal source 216 providesthe sampler 234 with a frequency modulated signal centered about afrequency of F 14. The samplers 226 and 234, shift registers 228 and240, the correlator 224 and a display 242 are coordinated in awell-known manner by signals provided by a clock 244. The display 242 issimilar to the display 30 of FIGS. 1 and 2 and provides a graphicalpresentation of the ocean bottom 28 and the pipe 26 as a function of thetime or distance traveled by the ship 18 of FIG. 1.

It is interesting to note that with respect to both the embodiment ofFIG. 2 and of FIG. 5 the reception of a signal at a frequency lower thanthat of the transmitted frequencies provides for a broader directivitypattern of received sonic energy than of the transmitted sonic energy,assuming that the transmitting and receiving transducers 22 and 24 ofFIG. 1 are of equal size. This is advantageous particularly in thesituation where the ship 18, while transporting the two transducers 22and 24, experiences a pitching and/or rolling motion induced by waves ofthe ocean 32. The relatively broad directivity pattern of the receivingtransducer 24 facilitates reception of echoes, such as echoes from thepipe 26, when a rolling of the ship 18 momentarily alters theorientation of the transducers 22 and 24 after a transmission of sonicenergy towards the pipe 26.

Also, with reference to both the embodiment of FIG. 2 and of FIG. 5, theuse of the frequency modulation may provide information relative to-theocean bottom 28 of FIG. 1 and pipe 26 which may not be asreadilyobtained with received sonic energy having a constant frequency. As iswell known, such modulation can provide a signature to received echoeswhich may aid in identifyinga reflecting object.

It is understood that the above-described embodiments of the inventionare illustrative only and that modifications thereof will occur to-thoseskilled in the art. Accordingly, it is desired that this invention isnot to be limited to the embodiments disclosed herein but is to belimited only as defined by' the appended claims.

What is claimed is: 1. in combination: means for transmitting signals ata plurality of frequencies into a media providing a nonlinearinteraction between a signal transmitted at a first frequency and asignal transmitted at a second frequency to provide a beam of radiantenergy propagating a signal at a frequency equal to the differencebetween one frequency of said plurality of frequencies and a secondfrequency of said plurality of frequencies, having acommon radiatingaperture for the transmission of said signals at said plurality offrequencies;

means coupled to said transmitting means for providing a referencehaving a frequency equal to said difference frequency; and

means responsive to said energy for correlating said signal at saiddifference frequency with said reference.

2. The combination according to claim 1 wherein said correlating meanscomprises:

means for providing said reference;

means for storing said reference in a first recirculating delay means;means for storing said difference frequency signal in a secondrecirculating delay means such that said stored difference frequencysignal processes relative to said stored reference; and

means responsive to said stored reference and said stored differencefrequency signal for providing a time of arrival of said differencefrequency signal at its storage means. i

3. The combination according 'to claim 1 wherein said correlating meanscomprises;

means for sampling said reference and said difference frequency signalto provide a series of samples of said reference and said differencefrequency signal; means for signaling the coincidence of a sample of oneof said series with individual samples of the other of said series; and

means for combining said coincidence signals to indicate a correlationof said reference and said difference frequency signal.

4. The combination of claim I further comprising means for displayingthe times of arrival at said correlating means of energy at saiddifference frequency to show the points of reflection in a media throughwhich said energy at said difference frequency propagates.

5. The combination of claim 4 wherein said transmitting means comprisesmeans for modulating an input signal to generate one of said transmittedsignals.

6. A sonic communication system comprising:

means for transmitting and means for receiving sonic energy signalstransmitted into a nonlinear medium, said transmitting means providingsonic energy at a first frequency and at a second frequency, saidreceiving means being responsive to energy at a frequency equal to thedifference of said first fre quency and said second frequency;

a source of signal having a frequency equal to said differencefrequency; and

means .for combining said signal at said difference frequency with asignal at higher frequencies to provide signals for transmission by saidtransmit- I ting means.

7. A sonic communication system comprising:

means for transmitting and means for receiving sonic energy signalstransmitted into a nonlinear medium, said transmitting means providingsonic energy at a first frequency and at a second frequency, saidreceiving means being responsive to energy at a frequency equal to thedifference of said first frequency and said second frequency;

a source of signal having a frequency equal to said differencefrequency;

means for combining said signal at said difference frequency with asignal at a higher frequency to provide a sonic energy signal fortransmission by said transmitting means; means for modulating saidsignal at said difference frequency; and I means for displaying thelocations of reflections of said energy at said difference frequencyfrom points of reflection in a medium through which said energy at saiddifference frequency propagates.

8. The system according to claim 7 wherein said displaying meanscomprises:

means for storing said signal of said source in a first recirculatingdelay means; means coupled to said receiving means for storing areceived signal in a second recirculating delay means such that saidreceived-signal processes relative to said stored signal of said source;and a means responsive to said stored received signal and said storedsignal of said source for providing a time of reception of said receivedsignal. 9. A signal source propagating a radiant energy signal through amedium remote from said signal source, said signal having a spectralbandwidth equal to substantially twice the center frequency of saidbandwidth, said source including a common radiator of radiant energy attwo frequencies, a frequency of said signal being equal to an arithmeticcombination of said two frequencies.

10. In combination: means for transmitting signals at a plurality offrequencies into an extended medium having a nonlinear propagationcharacteristic and which converts at least a portion of said signalsinto a signal having at least one frequency which is different than saidtransmitted frequencies, said transmitting means including a commonradiating aperture for radiation of said signals at said plurality offrequencies;

means coupled to said transmitting means for generating a referencehaving said one frequency;

means coupled to said medium for receiving said different frequencysignal; and

means for comparing a reference to said different frequency signal. 11.The combination of claim 10 wherein said transmitting means includesmeans for modulating said signals.

12. The combination according to claim 10 wherein said comparing meanscomprises:

means for sampling said reference and said different frequency signal toprovide a series of samples of said reference and said differentfrequency signal;

means for signaling the coincidence of a sample of one of said serieswith individual samples of the other of said-series; and

means for combining said coincidence signals to indicate a correlationof said reference and said different frequency signal.

13. The combination according to claim wherein said comparing meanscomprises:

means for providing said reference, said reference having a frequencyequal to said diflerent frequency;

means for storing said reference in ,a first recirculating delay means;

means for storing said different frequency signal in a secondrecirculating delay means such that said stored different frequencysignal processes relative to said stored reference; and

means responsive to said stored reference and said stored differentfrequency signal for providing a time of reception of said differentfrequency signal.

14. A system for communicating with a submerged reflector comprising incombination:

means forpropagating first signals through a first medium having anonlinear propagation characteristic for converting said first signalsinto at least one other signal; and

means for reflecting signals, said r'eflectin'g means being locatedwithin a second medium coupled to said first medium and having arelatively large transmissivity of said other signal and a relativelylow transmissivity of said first signals to enable said reflecting meansto selectively reflect said other signal while inhibiting the reflectionof said first signals.

15. The combination according to claim 14 further comprising:

means for providing a series of samples of a reference;

means for sampling said other signal to provide a series of samples ofsaid other signal;

means for signaling the coincidence of asample of one of said serieswith individual samples of the other of said series; and

means for combining said coincidence signals to indicate a correlationof said reference and said other .signal.

16. In combination:

means for transmitting atleast one directive beam of radiant energyhaving a plurality of frequencies into a medium having a nonlinearcharacteristic which converts at least a portion of such radiant energyinto radiant energy at a frequency lower than the frequencies of saidplurality of frequencies, said transmitting means having a commonradiatin g aperture for the radiation of said radiant energy at saidplurality of frequencies;

means for transporting said transmitting means through said mediumsequentially illuminate portions of said medium;

means located at a distance from said transmitting means for providingdata relative to the location of said transmitting means; and

means coupled to said medium for receiving at least a portion of saidlower frequency radiant energy, said receiving means having adirectivity pattern which is broader at said lower frequency than thedirectivity pattern of said transmitting means at said transmittedfrequencies.

12 17. The combination of claim 16 wherein a portion of said lowerfrequency radiant energy is reflected to said receiving means. 7

18. The combination of claim 17 wherein said receiving means is alsotransported by said transporting means, said broader directivity patternof said receiving means permitting reception of radiant energy as saidtransmitting beam is moved during said transportation.

19. In combination: means for transmitting energy at a plurality offrequencies into a first and a second medium, said first medium having anonlinear charactistic which converts said energy into energy at afrequency derived from a combination of frequencies of said plurality offrequencies, said second medium containing a reflector of said derivedfrequency energy to permit an echo of said derived frequency energy toemanate from said reflector, said transmitting means having a commonradiating aperture for the radiation of said energy at said plurality offrequencies; means located at a distance from said transmitting meansfor generating signals having data relative to the location of saidtransmitting means; and means responsive to said echo and coupled tosaid data signal generating means for indicating the location of saidreflector. 20. The combination of claim 19 wherein said indicating meanscomprises:

means for receiving said echo, said receiving means having apredetermined position relative to said transmitting means; and meansfor measuring the time of reception of said echo at said receiving meansrelative to the time of transmission of said energy at said plurality offrequencies. 21. The combination of claim 19 further comprising: meansfor transporting said transmitting means; and means for indicating thelocation of said transmitting means relative to a known point to providea map of said media. 22. The combination according to claim 19 whereinsaid location indicating means comprises:

means for providing a series of samples of a reference; means forsampling said echo to provide a series of samples of said echo; meansfor signaling the coincidence of a sample of one of said series withindividual samples of the other of said series; and means for combiningsaid coincidence signals to indicate the distance between said reflectorand said transmitting means. 23. A submerged reflector identificationsystem comprising in combination:

means for transmitting signals at a plurality of frequencies into amedium having a reflector therein for reflecting energy of said signals,said medium having a nonlinear characteristic for converting saidsignals into at least one other signal having a frequency derived from acombination of frequencies of said plurality of frequencies, saidtransmitting means having a common radiating aperture for radiation ofsaid signals at said plurality of frequencies;

25. The combination of claim 24 wherein said receiv- 7 ing meanscomprises means for comparing a replica of said modulation with saidreceived signals at said derived frequency.

26. In combination:

means for transmitting energy at a first frequency and at a secondfrequency into a medium which converts said energy into a first signalat afrequency equal to the difference of said first frequency and saidsecond frequency;

I means for providing a second signal at said difference frequency; 7

means for storing said first signal in a first recirculating delaymeans;

means for storing said second signal in a second recirculating delaymeans such that said stored second signal precesses relative to saidstored first signal; and 1 means responsive to said stored first signaland said stored second signal for providing the time of arrival of saidfirst signal at said first signal storage means.

I v i i i 1 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PatentNo. 3,786,405 Dated January 15, 1974 lnventcfls) Mark A. Chramiec andWilliam L. Konrad It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

'In the Claims Claim 2 column 9, line 30, change "processes" toprecesses Claim 8, column 10, line 29, change "processes" to precessesClaim 13, column 11, line 13, change "processes" to precesses Signed andsealed this 24th day of June 1975.

(SEAL) Attest:

C. MARSHALL DANN- RUTH C. MASON Commissioner of Patents AttestingOfficer and Trademarks UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTIQN Patent No. 3,786,405 Dated January 15, 1974 lnventcfls) MarkA. Chramiec and William L. Konrad It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

In the Claims Claim 2, column 9, line 30, change "processes" toprecesses Claim 8, column 10, line 29, change "processes" to precessesClaim 13, column 11, line 13, change "processes" to precesses Signed andsealed this 24th day of June 1975.

(SEAL) Attest:

, C. MARSHALL DANN- RUTH C. MASON Commissioner of Patents attestingOfficer and Trademarks FORM USCOMM-DC 60376-P69 UYS GOVERNMENT PRINTINGOFFICE: I969 0-365-334

1. In combination: means for transmitting signals at a plurality offrequencies into a media providing a nonlinear interaction between asignal transmitted at a first frequency and a signal transmitted at asecond frequency to provide a beam of radiant energy propagating asignal at a frequency equal to the difference between one frequency ofsaid plurality of frequencies and a second frequency of said pluralityof frequencies, having a common radiating aperture for the transmissionof said signals at said plurality of frequencies; means coupled to saidtransmitting means for providing a reference having a frequency equal tosaid difference frequency; and means responsive to said energy forcorrelating said signal at said difference frequency with saidreference.
 2. The combination according to claim 1 wherein saidcorrelating means comprises: means for providing said reference; meansfor storing said reference in a first recirculating delay means; meansfor storing said difference frequency signal in a second recirculatingdelay means such that said stored difference frequency signal processesrelative to said stored reference; and means responsive to said storedreference and said stored difference frequency signal for providing atime of arrival of said difference frequency signal at its storagemeans.
 3. The combination according to claim 1 wherein said correlatingmeans comprises: means for sampling said reference and said differencefrequency signal to provide a series of samples of said reference andsaid difference frequency signal; means for signaling the coincidence ofa sample of one of said series with individual samples of the other ofsaid series; and means for combining said coincidence signals toindicate a correlation of said reference and said difference fRequencysignal.
 4. The combination of claim 1 further comprising means fordisplaying the times of arrival at said correlating means of energy atsaid difference frequency to show the points of reflection in a mediathrough which said energy at said difference frequency propagates. 5.The combination of claim 4 wherein said transmitting means comprisesmeans for modulating an input signal to generate one of said transmittedsignals.
 6. A sonic communication system comprising: means fortransmitting and means for receiving sonic energy signals transmittedinto a nonlinear medium, said transmitting means providing sonic energyat a first frequency and at a second frequency, said receiving meansbeing responsive to energy at a frequency equal to the difference ofsaid first frequency and said second frequency; a source of signalhaving a frequency equal to said difference frequency; and means forcombining said signal at said difference frequency with a signal athigher frequencies to provide signals for transmission by saidtransmitting means.
 7. A sonic communication system comprising: meansfor transmitting and means for receiving sonic energy signalstransmitted into a nonlinear medium, said transmitting means providingsonic energy at a first frequency and at a second frequency, saidreceiving means being responsive to energy at a frequency equal to thedifference of said first frequency and said second frequency; a sourceof signal having a frequency equal to said difference frequency; meansfor combining said signal at said difference frequency with a signal ata higher frequency to provide a sonic energy signal for transmission bysaid transmitting means; means for modulating said signal at saiddifference frequency; and means for displaying the locations ofreflections of said energy at said difference frequency from points ofreflection in a medium through which said energy at said differencefrequency propagates.
 8. The system according to claim 7 wherein saiddisplaying means comprises: means for storing said signal of said sourcein a first recirculating delay means; means coupled to said receivingmeans for storing a received signal in a second recirculating delaymeans such that said received signal processes relative to said storedsignal of said source; and means responsive to said stored receivedsignal and said stored signal of said source for providing a time ofreception of said received signal.
 9. A signal source propagating aradiant energy signal through a medium remote from said signal source,said signal having a spectral bandwidth equal to substantially twice thecenter frequency of said bandwidth, said source including a commonradiator of radiant energy at two frequencies, a frequency of saidsignal being equal to an arithmetic combination of said two frequencies.10. In combination: means for transmitting signals at a plurality offrequencies into an extended medium having a nonlinear propagationcharacteristic and which converts at least a portion of said signalsinto a signal having at least one frequency which is different than saidtransmitted frequencies, said transmitting means including a commonradiating aperture for radiation of said signals at said plurality offrequencies; means coupled to said transmitting means for generating areference having said one frequency; means coupled to said medium forreceiving said different frequency signal; and means for comparing areference to said different frequency signal.
 11. The combination ofclaim 10 wherein said transmitting means includes means for modulatingsaid signals.
 12. The combination according to claim 10 wherein saidcomparing means comprises: means for sampling said reference and saiddifferent frequency signal to provide a series of samples of saidreference and said different frequency signal; means for signaling thecoincidence of a sample of one of said Series with individual samples ofthe other of said series; and means for combining said coincidencesignals to indicate a correlation of said reference and said differentfrequency signal.
 13. The combination according to claim 10 wherein saidcomparing means comprises: means for providing said reference, saidreference having a frequency equal to said different frequency; meansfor storing said reference in a first recirculating delay means; meansfor storing said different frequency signal in a second recirculatingdelay means such that said stored different frequency signal processesrelative to said stored reference; and means responsive to said storedreference and said stored different frequency signal for providing atime of reception of said different frequency signal.
 14. A system forcommunicating with a submerged reflector comprising in combination:means for propagating first signals through a first medium having anonlinear propagation characteristic for converting said first signalsinto at least one other signal; and means for reflecting signals, saidreflecting means being located within a second medium coupled to saidfirst medium and having a relatively large transmissivity of said othersignal and a relatively low transmissivity of said first signals toenable said reflecting means to selectively reflect said other signalwhile inhibiting the reflection of said first signals.
 15. Thecombination according to claim 14 further comprising: means forproviding a series of samples of a reference; means for sampling saidother signal to provide a series of samples of said other signal; meansfor signaling the coincidence of a sample of one of said series withindividual samples of the other of said series; and means for combiningsaid coincidence signals to indicate a correlation of said reference andsaid other signal.
 16. In combination: means for transmitting at leastone directive beam of radiant energy having a plurality of frequenciesinto a medium having a nonlinear characteristic which converts at leasta portion of such radiant energy into radiant energy at a frequencylower than the frequencies of said plurality of frequencies, saidtransmitting means having a common radiating aperture for the radiationof said radiant energy at said plurality of frequencies; means fortransporting said transmitting means through said medium sequentiallyilluminate portions of said medium; means located at a distance fromsaid transmitting means for providing data relative to the location ofsaid transmitting means; and means coupled to said medium for receivingat least a portion of said lower frequency radiant energy, saidreceiving means having a directivity pattern which is broader at saidlower frequency than the directivity pattern of said transmitting meansat said transmitted frequencies.
 17. The combination of claim 16 whereina portion of said lower frequency radiant energy is reflected to saidreceiving means.
 18. The combination of claim 17 wherein said receivingmeans is also transported by said transporting means, said broaderdirectivity pattern of said receiving means permitting reception ofradiant energy as said transmitting beam is moved during saidtransportation.
 19. In combination: means for transmitting energy at aplurality of frequencies into a first and a second medium, said firstmedium having a nonlinear charactistic which converts said energy intoenergy at a frequency derived from a combination of frequencies of saidplurality of frequencies, said second medium containing a reflector ofsaid derived frequency energy to permit an echo of said derivedfrequency energy to emanate from said reflector, said transmitting meanshaving a common radiating aperture for the radiation of said energy atsaid plurality of frequencies; means located at a distance from saidtransmitting means for generating signals having data relative to thelocatioN of said transmitting means; and means responsive to said echoand coupled to said data signal generating means for indicating thelocation of said reflector.
 20. The combination of claim 19 wherein saidindicating means comprises: means for receiving said echo, saidreceiving means having a predetermined position relative to saidtransmitting means; and means for measuring the time of reception ofsaid echo at said receiving means relative to the time of transmissionof said energy at said plurality of frequencies.
 21. The combination ofclaim 19 further comprising: means for transporting said transmittingmeans; and means for indicating the location of said transmitting meansrelative to a known point to provide a map of said media.
 22. Thecombination according to claim 19 wherein said location indicating meanscomprises: means for providing a series of samples of a reference; meansfor sampling said echo to provide a series of samples of said echo;means for signaling the coincidence of a sample of one of said serieswith individual samples of the other of said series; and means forcombining said coincidence signals to indicate the distance between saidreflector and said transmitting means.
 23. A submerged reflectoridentification system comprising in combination: means for transmittingsignals at a plurality of frequencies into a medium having a reflectortherein for reflecting energy of said signals, said medium having anonlinear characteristic for converting said signals into at least oneother signal having a frequency derived from a combination offrequencies of said plurality of frequencies, said transmitting meanshaving a common radiating aperture for radiation of said signals at saidplurality of frequencies; means for receiving signals at said derivedfrequency reflected from said reflector; means connecting with saidtransmitting means for providing a modulation of said derived frequencyfor identification of said reflector; and means coupled to saidreceiving means and to said modulation means for comparing a modulationof said received signals with said transmitted modulation.
 24. Thecombination of claim 23 further comprising means connecting with saidreceiving means for displaying said received signals at said derivedfrequency.
 25. The combination of claim 24 wherein said receiving meanscomprises means for comparing a replica of said modulation with saidreceived signals at said derived frequency.
 26. In combination: meansfor transmitting energy at a first frequency and at a second frequencyinto a medium which converts said energy into a first signal at afrequency equal to the difference of said first frequency and saidsecond frequency; means for providing a second signal at said differencefrequency; means for storing said first signal in a first recirculatingdelay means; means for storing said second signal in a secondrecirculating delay means such that said stored second signal precessesrelative to said stored first signal; and means responsive to saidstored first signal and said stored second signal for providing the timeof arrival of said first signal at said first signal storage means.